During the quest A Ghost from the Past, you and Green-Venom-Tongue discover that Lyra was tasked with carrying out the purification of the Sanctuary. Subsequently, she was ordered to kill Primate Jonas, the predecessor to Primate Artorius. After that, Artorius offered Lyra to become his champion, the Black Dragon - an offer Lyra accepted. In The Wrath of Sithis, Astara will ask you to travel to Knightsgrave and ensure that Lyra is killed. In Knightsgrave, the Wrath of Sithis will appear and consume Lyra's soul.